In the past, media systems have been composed of a single player device, such as a stereo system, with multiple speakers attached by long analog lines. In such prior art systems, the possible configurations are limited by the use of a single player system and the number of speakers or other presentation devices that the single player can drive. Also, because the presentation is defined by the hardwired speaker configuration, it is difficult to alter the physical configuration without extensive rewiring.
As a solution to the constraints imposed by prior art single-player systems, the present invention employs a digital streaming paradigm with multiple players, referred to herein as output devices. The primary source for streaming media is referred to as the media server. By supporting multiple players or output devices, streaming media systems provide the ability to support greater numbers of speakers, displays or other presentation devices, as well as much improved configurability. Such streaming media systems are particularly useful for delivering media playback to multiple rooms, listening areas separated by distance or otherwise spatially distinct media delivery points.
However, by implementing multiple playback devices, time-related distortion may be experienced where two listening or viewing areas overlap. This time-related distortion may result from lack of playback synchronization between output devices.
The rate at which each player device processes media data (e.g., by converting digital samples to analog signals for speakers, etc.) is determined by the frequency of the player device's internal crystal that is used to provide the primary clock source. Yet, crystal oscillator frequencies may vary from crystal to crystal, and are also affected by crystal age and temperature. Therefore, the frequencies for two crystal oscillators (and thus the playback rates for two media player devices) may differ by an initial frequency error that may also vary over time as the crystal ages or the playback environment changes. The scenario described below will help illustrate how such synchronization errors arise, and what problems are associated with such errors.
As an example of a streaming system, consider the following family residence scenario. Assume that a first output device is a portable wireless device carried by Daughter, who happens to be relaxing in the living room while listening through headphones to her favorite CD streamed from a first media server. At the same time, Son A and Son B are watching the newest DVD movie streamed from a second media server to a second output device in the family room, which feeds the DVD video to a big screen television and analog audio to surround sound speakers.
While Son A, Son B and Daughter are warm inside, Father is outside on the back porch barbecuing in near-freezing weather. The back porch is wired with an older third output device, which drives the outdoor speakers. Initially, Father is listening to the streaming audio from the first media server, but finds that Daughter's choice in music is not to his taste. Father then switches to the DVD media stream in progress so that he can watch the big screen television through the window, while listening to the analog audio played back over the outdoor speakers.
Without synchronization of the media stream playback, the audio that Father is listening to on the porch may begin to diverge from the DVD playback occurring on the independent output device in the family room. This may occur, for example, because the crystal inside the output device on the back porch is older and colder than the crystal inside the output device in the family room, which is stored in a cabinet already heated well-above room temperature by other similarly-stored electronic equipment. Also, because Father joined a media stream already in progress, the start-up latency of the outdoor output device may cause a playback timing offset relative to the output device in the family room.
For Son A, who is sitting near the porch window, the time discrepancy between the audio played in the family room and the muffled audio coming through the window from the porch speakers may diminish his enjoyment of the DVD movie, as the porch audio interferes with the surround sound audio inside. If the time divergence is sufficiently large, Father may also notice with dismay that the sound track playback on the porch is out of sync with the events occurring on the big screen television inside. These types of problems may be more apparent for longer playing media streams.
In view of the foregoing problems with streaming systems, the present inventor has determined that it would be desirable to have a streaming media system in which the playback processes of the various output devices are synchronized.